The goals are to determine if during lactation, plasticity in oxytocin (OT) neurons involves the proto-oncogene c-fos, and if this gene expression correlates with a rise in OT in cerebrospinal fluid (CSF). The physiological consequences of plasticity and whether they require OT or gamma-aminobutyric acid (GABA) will also be studied. This research has implications for reproduction, endocrinology and cancer. Plasticity is the formation of new and reversible synapses involving growth and adaptation of neurons. OT neurons in the magnocellular system undergo plasticity from mid-gestation through lactation. These changes in synaptic morphology can be produced by intracerebroventricular (icv) infusion of OT and are believed necessary for the synchronized firing of OT neurons during suckling. Expression of c-fos is postulated in synaptic remodeling of the OT neuron during lactation since this proto-oncogene has been associated with plasticity of other neural systems. We will examine, using conscious rats, whether changes in OT levels in CSF correlate with expression of c- fos or its encoded protein, fos, in OT neurons during gestation, lactation or suckling, or after infusion (icv) of OT. It is proposed further, that OT-induced changes in plasticity decrease the responsiveness of the OT neuron to osmotic stimulation during lactation by a mechanism involving the inhibitory neurotransmitter, GABA. New synapses on the OT neuron during lactation contain GABA and the GABA receptor protein increases in cells when expression of c-fos is stimulated by Metrazole. Therefore, using receptor agonists and antagonists we will examine whether the attenuated release of OT to osmotic stimulation during lactation requires OT or GABA. Also, we will determine in anesthetized virgin or lactating rats whether GABA or its antagonist alters the firing rate of OT neurons to osmotic stimulation or the threshold and maximal frequency at which OT is released by electrical stimulation of magnocellular neurons. Techniques will include: RIA of OT and vasopressin, push-pull perfusion of CSF, electrophysiological recording and stimulation of the magnocellular system, Northern blot, slot blot and in situ hybridization for assessing mRNA for c-fos, and immunoblot and immunocytochemistry for identifying fos protein.